1. Field of the Invention
The present invention relates to a liquid crystal device for use, for example, in a display device, an optical shutter array, etc., and more particularly, to a liquid crystal device which employs a ferroelectric liquid crystal having two bistable orientation states in the chiral smectic temperature range. In particular, the present invention concerns a novel liquid crystal device which features improved orientation uniformity, durability and contrast between bright and dark states.
2. Description of the Related Art
Display devices which control the intensity of light which passes through them by means of a polarizer-liquid crystal-polarizer assembly and the refractive anisotropy of ferroelectric liquid crystal molecules are known. Such display devices have been proposed in the specifications of, for example, Japanese Patent Laid-Open No. 56-107216, U.S. Pat. No. 4,367,924 to Clark et al and so on. The ferroelectric liquid crystal of the mentioned above type is generally in the chiral smectic C (SmC*) or H (SmH*) phase in a particular temperature range. By disposing the helically structured chiral smectic C or H liquid crystal between a pair of substrates separated by a distance sufficiently small to ensure that the helix unwinds, the liquid crystal orientation field is found to possess two stable optical states. The bistable ferroelectric liquid crystal can be switched between its two stable states by reversing the polarity of an externally applied electric field. Also, since the liquid crystal can maintain an image in the absence of the electric field, it therefore functions as a memory. The liquid crystal is characterized by its high-speed response to changes in the electric field, which enables the liquid crystal to be applied to high-speed memory-type display devices.
In order to manufacture an optical modulation device using the ferroelectric smectic liquid crystal with a bistable orientation field and which exhibits memory and high-speed response characteristics on a commercial basis, the two stable states must be present both uniformly and stably. Moreover, the optical modulation device must have excellent durability, as well as high contrast between its bright and dark states
Okada et al discloses in U.S. Pat. No. 4,639,089 a ferroelectric smectic liquid crystal having a temperature range achieving the cholesteric phase which is utilized in a liquid crystal device provided with a uniaxial orientation processing axis obtained by a rubbing or an oblique deposition process. However, the ferroelectric smectic liquid crystal device which exhibits uniform bistable characteristics achieved by the rubbing or oblique deposition processes is sometimes observed to pass less light in a bright state than the liquid crystal device proposed by N. A. Clark et al.
In the aforementioned ferroelectric smectic liquid crystal device exhibiting uniform bistability, the uniform bistability requires that the liquid crystal molecules be aligned in a highly ordered state. Such highly ordered molecular orientation states are easily affected by external stress such as impact or strain, and if the ordered molecular orientation is disrupted, the liquid crystal generates a sanded texture. The generation of a sanded texture upon impact is described in, for example, U.S. Pat. No. 4,674,839 to Tsuboyama et al.
The present inventors conducted various experiments and determined that, although use of a sufficiently thick (for example, 600 .ANG. or more) high polymeric organic film as an orientation controlling film ensures a sufficiently uniform molecular orientation over a wide area, it nevertheless deteriorates the driving characteristics of the liquid crystal device in which it is installed. It has been proposed in Japanese Laid-Open Patent No. Sho 62-52528 that ferroelectric liquid crystal bistability can be achieved by using a high polymeric organic film having a thickness of at most 200 .ANG.. Although it is preferred to use a thin high polymeric organic film as the orientation controlling film from the viewpoints of attaining excellent driving and bistable characteristics, the thin high polymer film causes defects in the molecular orientation, precluding the attainment of uniformly oriented molecules. Of course, a liquid crystal with molecular orientation defects does not exhibit excellent bistability.
The present inventors have also conducted experiments and determined that when a large black portion and a large white portion are displayed on a display panel (as shown in FIG. 18 (A)) and the display is switched so that the panel only displays white, the area of the panel which previously displayed black had a lower transmittance (i.e., remained in a light gray state) than did the previous white display portion. See FIG. 18 (B). Additionally, since the gray portion is maintained for a relatively long period of time, when the new contents were displayed, the newly displayed contents were superimposed over the previously displayed light gray portion. This is called "burning" of the previously displayed contents The burning phenomenon provides for a display which is difficult for an operator of, for example, a word processing device to view.